This invention relates generally to antennas and, more particularly, to a small size, high efficiency dual band antenna system that includes both a single reflector low frequency antenna and a dual reflector high frequency antenna and that is constructed in a manner that minimizes squint beam pointing error between the two frequency bands.
Conventional satellite communications applications require the use of highly directional dual band antennas to transmit and receive microwave signals between orbiting satellites or between an orbiting satellite and a ground-based uplink. In applications requiring high antenna efficiency, a dual-band antenna system using two separate antennas and two separate antenna feeds may be used. However, such a system is impractical in the above-mentioned satellite communications applications, as it is expensive to implement due to component redundancies and requires a relatively large amount of real estate.
Low data rate link antenna systems such as those with a conventional Cassegrain geometry offer one solution to the above-mentioned real estate and cost issues associated with the dual antenna/dual feed configuration. In a Cassegrain antenna system, a parabolic reflector acts as the primary reflector, and a smaller hyperbolic subreflector deflects incoming microwaves to a signal feed located at or near the center of the reflector. However, the subreflector can only be about 5 wavelengths (relative to the lower frequency band) in size due to the size of the dual frequency antenna aperture in a Cassegrain antenna system. This small size results in high subreflector diffraction loss for the lower frequency band. Consequently, such a system is too inefficient for most commercial and military satellite applications.
A low data rate link antenna having a small single reflector with a dual frequency feed offers another commercially viable solution to the above-mentioned real estate and cost issues associated with the dual antenna/dual feed antenna systems. However, as with the Cassegrain antenna system, a single reflector/dual frequency feed antenna system is complex to build and is inefficient due to high feed insertion loss. In addition, a single reflector/dual frequency feed system has a high associated defocus loss caused by phase center offset even though the horn shape dimensions are mechanically the same for both the low and the high frequency bands but electrically different due to the different wavelengths between the two bands. For circular polarization, this antenna has offset beam squint pointing error between the high and low frequency bands because the beam squint is frequency dependent even though the offset reflector has the same focal point for both frequency bands. Consequently, such an antenna system typically has less than 50% antenna efficiency and a high axial ratio for each frequency band, and is limited in its RF high power capability.
Accordingly, an object of the present invention is to provide a high efficiency dual band antenna system that is small in size and that has a feed design that is of minimal mechanical complexity.
A further object of the present invention is to provide a high efficiency dual band antenna system that is constructed so that both the high and the low frequency bands have optimum phase centers and so that the gain of the low and high frequency feeds is maximized.
A further object of the present invention is to provide a high efficiency dual band antenna system in which both antennas have low feed loss and good axial ratio, and in which the antennas are configured to eliminate beam squint pointing error between the high and low frequency bands.
Accordingly, the present invention provides an antenna system that includes a paraboloidal main offset reflector for reflecting a low frequency signal as well as a high frequency antenna including both the main offset reflector and a hyperboloidal subreflector for reflecting a high frequency signal discrete from the low frequency signal. The hyperboloidal subreflector includes a frequency selective surface for passing the low frequency signal reflected by the paraboloidal main offset reflector.
The antenna system according to the present invention is highly efficient and small in size compared to the above discussed conventional antenna systems, as the hyperboloidal subreflector essentially acts as a lowpass filter to transmit the low frequency signal with no subreflector diffraction loss and is highly reflective at the high frequency. Also, offset beam squint pointing error can be eliminated because the high and low bands have separate feed focal locations. In addition, overall system cost is reduced because, for example, loose tolerance smooth feed horns are used rather than the tight tolerance corrugated feed horns required in dual band antenna systems.